1. Field of the Invention
The invention relates to a device for protecting a ballast circuit from excess voltages. Such devices are e.g. used in combination with ballast circuits for gas discharge lamps.
2. Discussion of the Background of the Invention and Material Information
In electronic ballast circuits for sine-shaped input current, boost converters are used comprising a capacitor as a storage element. In such devices a starting resistor is provided for limiting the input current during start-up. Once the capacitor is loaded, the starting resistor is shunted by a relay, a triac or a thyristor. Such start-up current limiters are described, for example, in the Siemens application notes PD22 8905 and PD22 9002, published by Siemens AG of Munich, Germany as well as in U.S. Pat. No. 5,032,968. A disadvantage of these circuits is the electric loss during normal operation caused by the voltage drop over the shunting element. This loss is the product of this voltage drop times the input current. A further disadvantage, when using a triac or a thyristor, is that these devices can only be opened when no current flows through them. Therefore, they cannot be opened for disconnecting the ballast if an excess voltage should be detected.
The description of the pre-regulator UC 3854 of the Unitrode Integrated Circuits Corporation of Merrimack, N.H., on pages 6-14, discloses the buck-topology for input current limitation with pulse width modulation. The control circuits for pulse width modulation are, however, complicated. After termination of the start-up procedure during normal operation the switch, which was periodically opened and closed during start-up for limiting the current, is closed permanently. No further protection of the capacitor and the following load is provided if a transient or permanent excess voltage should be applied to the device.
European Patent Publication No. EP 339 598 describes a protective circuit for capacitive loads comprising a control arrangement of comparators for detecting an excess voltage or an excess current. The control arrangement drives a switch for disconnecting the load when the current or the voltage exceed their maximum values. This circuit is expensive because it requires a large number of components, e.g. several amplifiers and logic circuits.